For underwater weeds, if you aren't going to use chemicals the only three other options are mechanical removal, stop the sunlight from reaching the pond bottom or triploid grass carp if the weeds are of the type that they will eat.


www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).